If you’ve watched recent action movies, you may have noticed a slightly disorienting jerkiness to the video. No, it’s not because of shaky cam and excessive jump cuts. Many modern (and some older) movies have an effect called “strobing” that makes action scenes look less fluid than the others. Today, we’re going to explain why this happens.

What Is Strobing?

Strobing or jerkiness occurs when the frames of a movie don’t quite blend together well enough, creating an effect that’s a bit like looking at a moving object under a very fast strobe light. This can occur because each frame doesn’t have enough motion blur (which we’ll explain later on) to blend each frame to the next, or if there aren’t enough frames to make smooth motion to begin with. Depending on the severity of the effect, some people may not notice it, but if it’s really bad, it can be distracting. (This is not to be confused with judder from 3:2 pulldown, which is a separate thing entirely—and usually far less noticeable.)

To see what strobing looks like in a movie, we’ll use Captain America: Civil War as an example. Take these two scenes, both of which show Tony Stark moving his head around as he talks to Steve Rogers. We’ve reduced the film clip to a GIF, so it won’t be as detailed as your Blu-Ray at home, but you can still see that Tony and Steve’s movement as they talk is pretty smooth.

Compare this to a later scene where Steve and Tony are arguing yet again. However, this one takes place just before the big airport action scene. Once this scene starts, the motion starts to look choppier. The motion as Tony turns his head and shouts at Steve looks just a bit less smooth. Again, since this is a GIF, it may not be as detailed, but the choppiness of the video is still noticeable.

This effect gets even more exaggerated in this shot with Tony and Peter Parker. Peter flails his arms and Tony has to grab him to calm him down. The more the characters move, the choppier the footage looks.

Frame Rate and Motion Blur, Explained

To understand why this effect occurs, we need to explain a bit about how movies work. Every film, TV show, YouTube video, or animated GIF you watch is actually a series of still images playing in quick succession. Play enough continuous frames rapidly, and your eye sees them as motion. Most movies (with rare exceptions) are shot in 24 frames per second (or fps). That means for every second of footage, you’re actually seeing 24 still images, each one only slightly different than the last.

The more frames you see per second, the smoother the motion will look. The image below demonstrates how higher frame rates create smoother motion. It’s not a perfect representation, but as you can see, the top line flows from one side of the screen to the other smoothly. The middle line looks like it’s sliding across, but it’s a little jittery. The bottom line doesn’t look like it’s moving at all. It looks like it’s repeatedly jumping from one spot to another.

Sometimes, a director can manipulate the frame rate on purpose for a certain effect. For example, in Mad Max: Fury Road, director George Miller would speed up or slow down the frame rate on particular shots in order to make the action more or less choppy, depending on what the scene needed at the time. This now-famous shot, for example, has a lot of jerkiness, but it’s for a good reason. Nux is driving into a dust storm, with lightning flashing over his face. If ever there was a reason to adjust your frame rate on purpose to get choppier motion, this is it.

The number of frames per second is only part of the illusion of motion, though. Objects and people still move between frames. When a camera captures an object that’s moving, it creates a motion blur. The faster the movement, the more blurry an object looks (just like when you take a normal still photo). When you see all the frames of a movie, this blur looks like continuous motion because your eyes can’t track fast-moving objects well. However, when you look at a single frame of a video where an object is moving quickly, it looks a bit like this:

Take this one frame on its own, and it looks like Spider-Man is growing a second head and has eight fingers on his left hand. You don’t notice that this particular frame is blurry because its just one of 24 frames you saw in that particular second of film, but your brain recognizes that blur as motion.

How Directors Can Manipulate Frame Rate and Motion Blur to Create Strobing

Motion blur and frame rate are tightly connected. You can see how that interplay works with this interactive tool. By default, that link will show you two balls sliding across the screen. One will show what 60fps looks like, the other is 25fps. As you might expect, the ball moving at 25fps is a lot blurrier. Both objects are moving at the same speed, but the ball that’s being “recorded” at 60fps has a shorter distance to travel in each frame, so it’s less blurry in a single image.

If you shoot an action scene at, say, 48fps, but then play it back at 24fps at normal speed, the film will essentially skip every other frame each second. The result is that each frame will have less motion blur, making the footage look slightly choppier than the other scenes that were shot at 24fps to begin with. To see what this looks like, open the interactive tool again. This time set both balls to 24fps, but change the motion blur on one of them to “0.5 (Light).” Even though both of them are rendered at the same frame rate, the one with less motion blur will look choppier. This is one way what the Russo brothers could have gotten the choppiness in the Civil War clips from earlier. On the days they shot the airport scenes with the special cameras, they could have shot at 48fps (or higher) and reduced the number of frames per second included in the final shots, resulting in choppier motion.

There are other ways to affect the motion blur of an image, too. While shooting Saving Private Ryan, director Steven Spielberg used a high shutter speed while shooting action sequences. Shutter speed determines how much light is exposed to the film per frame. By opening and closing the shutter faster than usual, the camera captures less light and thus less movement per frame. This reduces the motion blur without shooting at a different frame rate. This was done intentionally to give the film a shakier, more unstable feel which fit the chaos of the scene while storming Omaha beach.

Whether a director shot their film in a higher frame rate from the start like in Captain America: Civil War, manipulated the frame rate on a per shot basis like in Mad Max Fury Road, or if they used a higher shutter speed like in Saving Private Ryan, the result is the same. There’s less motion blur on each frame of the movie, which makes the movement not quite perfectly smooth. Your brain registers that lack of smoothness as a jerkiness that doesn’t feel quite right.

Interestingly, this is the opposite problem you see with the so-called “soap opera effect.” That effect occurs when your TV tries to automatically add extra frames and motion blur to video and ends up making movies look unnaturally smooth. Unfortunately, while you can usually turn off your TV’s auto-smoothing features, you can’t do much about choppy movies. In the end, the choppiness is (usually) a style choice and any attempt to “fix it” will only make it look worse. However, the next time you see your movie suddenly get jerky, at least you know an action scene is coming up, so you should stay in your seat.